Miniaturized ion mobility spectrometer

ABSTRACT

By utilizing the combination of a unique electronic ion injection control circuit in conjunction with a particularly designed drift cell construction, the instantly disclosed ion mobility spectrometer achieves increased levels of sensitivity, while achieving significant reductions in size and weight. The instant IMS is of a much simpler and easy to manufacture design, rugged and hermetically sealed, capable of operation at high temperatures to at least 250° C., and is uniquely sensitive, particularly to explosive chemicals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present specification is a continuation of U.S. patent applicationSer. No. 11/760,388, which claims benefit of the filing date of U.S.Provisional Patent Application No. 60/812,463, filed on Jun. 9, 2006,the contents of both of which are herein incorporated by reference intheir entirety.

Certain aspects of this invention were made with Government supportunder contract numbers NAS2-03137 and NNAO4CAO8C awarded by NASA. TheGovernment has certain rights in those aspects of the invention.

FIELD OF THE INVENTION

This invention relates to ion mobility spectrometry; particularly to ionmobility spectrometers having enhanced sensitivity and performance forthe detection of trace chemicals; and most particularly to a highlysensitive and miniaturized Ion Mobility Spectrometer (IMS) incorporatinga plurality of mechanical and electrical innovations, resulting insynergistic operability enhancements.

BACKGROUND

The Ion Mobility Spectrometer was invented by Dr. Martin J. Cohen andothers in the late 1960's at Franklin GNO Corporation in West PalmBeach. The genesis of this idea resulted from Dr. Cohen's interest ingaseous electronics and radiation physics. The original patent for IMS,U.S. Pat. No. 3,699,333 was filed in October 1968, and granted Oct. 17,1972. This patent discloses the concept of “Plasma Chromatography”, anearly name for IMS and describes the instrument concept and shows aspectrum. This patent was followed by a number of others that describerefinements and expansions of the original IMS concept and instrumentdesign, and discuss a variety of applications and analyticalmethodologies. These patents, all assigned to Franklin GNO, are: U.S.Pat. Nos. 3,593,018; 3,621,239; 3,621,240; 3,624,389; 3,626,178;3,626,179; 3,626,180; 3,626,181; 3,626,182; 3,629,574; 3,668,382;3,668,383; 3,668,385; 3,697,748; and 3,697,749. U.S. Pat. No. 3,845,301granted Oct. 29, 1974, describes the design and functioning of an IMSvery similar to those used to the current day, with the exception of thespecific method of detecting and observing the ion peaks.

IMS has military and anti-terror utilities for the detection of chemicalwarfare (CW) agents and explosives, for which the instantly discloseddevice is uniquely capable. The US and UK governments have purchasedinstruments for use in the area of CW agent detection, in particular.

Under government supported contract research, primarily for the FAA forexplosives detection, and for NASA for a unique methodology using IMSfor planetary atmosphere analyses, basic technology currently used atairports for trace vapor detection of concealed explosives wasdeveloped. The NASA work produced instrumentation that was capable ofproviding trace component analysis of the atmospheres of Mars, Titan,and comets. This methodology was commercialized for the analysis ofultra-high purity gases for the semiconductor industry.

Patents for an explosive detection application and for the pure gasanalysis application were issued: U.S. Pat. No. 5,162,652 granted Nov.10, 1992, and U.S. Pat. No. 5,457,316 granted Oct. 10, 1995. A number ofpure gas analysis patents, both US and international, have been issued,e.g. U.S. Pat. No. 6,740,873 issued May 25, 2004, and U.S. Pat. No.6,895,339 issued May 17, 2005. The instant inventors have targetedcommercial and government applications that require a rugged,dependable, miniature Ion Mobility Spectrometer. The initial objectivewas to concentrate on the explosive detection market which provides thegreatest opportunity for the instantly disclosed unique miniaturizedIMS. The instant inventors developed a hand-held detector for traceexplosives detection. However, the focus of the NASA SBIR was tocontinue the application of IMS for planetary atmosphere analysis inwhich the rugged hermetically sealed miniaturized design was importantto reduce weight and consumables usage. Out of this work, anotherimportant prototype commercial application for pure gas analysis hasalso been developed.

U.S. Pat. No. 3,593,018, issued to Cohen on Jul. 13, 1971 is directedtowards an apparatus and method for sorting and detecting ions in adrift cell, the electric fields applied to the cell being controlled atappropriate times to minimize dispersion of bunched ions produced by apulsed source. Bunched product ions produced by ion-molecule reactionsare analyzed in accordance with their velocity in a drift field.

U.S. Pat. No. 3,621,239, issued to Cohen on Nov. 16, 1971 deals withmethods for sorting and detecting trace gases which undergo ion-moleculereactions. Particular species of reactant ions are selected by choice ofreagent gas and/or by reactant ion filtering to produce predictableproduct ions by reaction with trace gas molecules of a sample. Thesample may be reacted with different species of reactant ions and theresults compared to confirm the presence of particular species ofproduct ions. The reagents producing different species of reactant ionsmay have ionization potentials above and below the ionization potentialof the expected trace gas molecules.

U.S. Pat. No. 3,624,389 issued to Cohen et al. on Nov. 30, 1971, isdirected toward an apparatus and methods for sorting and detecting tracegases which undergo ion-molecule reactions. Positive or negative ions ofthe trace gas are formed by ion-molecule reactions between the moleculesof the trace gas and primary ions from another gas. Ions are classifiedin accordance with their velocity in a stream of gas while subjected toan electric drift field.

U.S. Pat. No. 3,626,180 issued to Carroll et al. on Dec. 7, 1971, isdirected towards apparatus and methods for sorting and detecting tracegases which undergo ion-molecule reactions, trace ions being formed in areactive gaseous medium and being analyzed in a nonreactive gaseousmedium. The ions are classified in accordance with their velocity in anelectric drift field.

U.S. Pat. No. 3,626,182 issued to Cohen on Dec. 7, 1971, and is directedtowards an apparatus and method for sorting and detecting ions in adrift cell, the electric fields applied to different regions of the cellbeing controlled at appropriate times to ensure the rapid withdrawal ofthe ions from a reaction region to an analysis region, the bunching ofthe ions in the analysis region, and thereafter the separation of thebunched ions in accordance with ion drift velocity, and detection ofseparated ion species.

U.S. Pat. No. 3,629,574 issued to Carroll on Dec. 21, 1971, deals with aprocess wherein electrons are separated from ions by subjecting thesecharged particles to a drift field to cause them to move from a firstregion toward a second region and by interposing an electron filter inthe drift field between said regions, the filter comprising a pair ofgrid members to which high-frequency alternating voltages are applied.This principle is applied to an electron capture detector and to adevice which separates and detects ions in accordance with theirmobility.

U.S. Pat. No. 3,697,748 issued to Cohen on Oct. 10, 1972 is directedtoward a process wherein response time of drift-cell apparatus formeasuring trace gases is improved by heating the drift cell walls and/orthe sample inlet to reduce the accumulation of sample substances. Heatedfilters and electrode structures with tortuous gas paths are alsodisclosed.

U.S. Pat. No. 3,697,749 issued to Wernlund on Oct. 10, 1972, is directedtoward detection of small-source plumes, as by an airborne instrument,wherein ions formed from the molecules of a gaseous sample and collectedby the airborne instrument are segregated in a drift field, and signalsproduced in response to the detection of the segregated ions areseparated into short-duration plume signal components and long-durationbackground components. The short-duration components are indicated withenhanced resolution.

U.S. Pat. No. 3,699,333 issued to Cohen et al. on Oct. 17, 1972, isdirected towards an apparatus and method for sorting and detecting tracegases which undergo ion-molecule reactions. Positive or negative ions ofthe trace gas are formed by ion-molecule reactions between the moleculesof the trace gas and primary ions from another source. Ions areclassified by selective ion gating according to their velocity in anelectric drift field.

U.S. Pat. No. 3,845,301 issued to Wernlund et al. on Oct. 29, 1974, isdirected toward a process wherein plasma chromatograph response time isdecreased by improvement of the gas low. An ion-molecule reaction regionis provided in tandem with a larger diameter drift region, and a gasoutlet is provided at the junction of the regions. Sample gas flowingthrough the ion-molecule reaction region into the drift region isre-directed by a counter-low of drift gas through the drift region,causing both gases to exit through the outlet and reducing intrusion ofthe sample gas into the drift region. Diffuse gas flow is employed inboth regions, special structures being provided to avoid gas jetting.

U.S. Pat. No. 4,855,595 issued to Blanchard on Aug. 8, 1989, disclosesan ion mobility spectrometer for detecting ions and for facilitatingcontrolled chemical reactions is described incorporating an inlet forcarrier and sample gas, a reaction region having an ionization sourceand at least two electrodes for generating an electric field and a driftregion having at least two electrodes for generating an electric fieldtherein wherein each electrode is coupled to a power supply for placinga predetermined potential on the electrode and wherein each power supplyis controlled by an electric field controller for providing a sequenceof potentials on each electrode in the reaction region and drift regionto control the motion and position of ions in the drift region. Theinvention claims to overcome the problem of detection sensitivity,detection selectivity and resolution between ions of similar mobility;however enablement of a gridless system is neither taught nor disclosed.

In a later published paper entitled “Ion Injection Mobility SpectrometerUsing Field Gradient Barriers, i.e. Ion Wells (Blanchard et al, IJIMS5(2002)3, Pp 15-18), a gridless system is disclosed. Blanchard requiresthe use of a dual zone system for creating a “Trigger well” and a“Storage Well” which must manipulate the voltages at two rings in orderto provide an ion reservoir.

These disclosures further illustrate the inability of skilled artisanssuch as Blanchard and his colleagues to constructively or actuallyreduce to practice a miniaturized IMS device having highly enhancedsensitivity and performance for the detection of trace chemicals,particularly the relatively high molecular weight, low vapor pressureexplosive chemicals.

U.S. Pat. No. 5,162,652 issued to Cohen et al. on Nov. 10, 1992, isdirected towards an apparatus and method for the detection andidentification of the presence of chosen molecules, typically toxic orcontraband located within sealed luggage and the like, comprisessubjecting the sealed luggage to a process whereby a portion of theenclosed atmosphere within the luggage is extracted and combined withthe surrounding atmosphere in a closed chamber. The extracted, combinedsample is passed to a collector, typically a molecule adsorber, whichconcentrates the chosen molecules by collection on a collecting surface.After the end of a collection period, the adsorbed molecules arereleased from the surface and passed to an identifier, such as an ionmobility spectrometer. By use of appropriate collection and valvingelements, analysis can be accomplished quickly and accurately for alarge number of luggage items or the like subject to examination.

U.S. Pat. No. 5,200,614 issued to Jenkins on Apr. 6, 1993, describes anion mobility spectrometer which employs an electron capture process. Asample gas stream is irradiated to produce positive ions and electronsin an ionization chamber. An open grid electrode is employed in theionization chamber to maintain a field-free space that claims to allowion population to build up in the ionization chamber. However, a highelectric field is periodically generated across the ionization chamberfor periods of less than one millisecond to cause most ions of onepolarity in the ionization chamber to be swept out and into a driftchamber. Ions of opposite polarity are discharge on the walls of theionization chamber. The ions entering the drift chamber travel at driftvelocities dependant on their respective charge and mass. A collectorelectrode is provided for sequentially collecting ions of differingmass, and the collected ion current is transmitted to a signalprocessing means for measuring intensity and arrival times for thecollected ions. A potential can be maintained between the drift chamberand the ionization chamber for preventing ions from traveling down thedrift chamber. However, this potential between the drift chamber and theionization chamber may periodically be switched synchronously with thegeneration of the field across the ionization chamber to enable ions topass into the drift chamber during the switching.

U.S. Pat. No. 5,457,316 issued to Cohen et al. on Oct. 10, 1995 relatesto an ion mobility spectrometer sensor apparatus which is enclosed in aseparate hermetically sealed housing, utilizing a drift gas for thedetermination of trace contaminants in a carrier gas, including acontainer for a sample gas containing an analyte the concentration ofwhich is to be determined, means for purifying the sample gas to producethe carrier gas from it, the means for purifying being hermeticallyconnected from the container through a metallic pipe, a source for thepurified drift gas which may be the same or different than the carriergas, an ion mobility spectrometer sensor having a carrier gas entranceand a drift gas entrance and a gas exit, the ion mobility spectrometersensor being hermetically connected by a metallic pipe from thepurifying means and from the source of the drift gas, a back diffusiontrap is hermetically connected from the gas exit, and a signal readoutis electrically and hermetically connected from the ion mobilityspectrometer sensor for electrically sensing and displaying signalsobtained in the sensor.

U.S. Pat. No. 6,606,899 issued to Ketkar et al. on Aug. 19, 2003describes a device for measuring a total concentration of impurities ina sample gas which includes a housing having an inlet to allow thesample gas to enter the housing, an emitter to generate ions from thesample gas, a field gradient to accelerate the ions toward a collector,the collector adapted to measure total ions, and an outlet to allow thesample gas to exit the housing, whereby a change in total ions receivedby the collector indicates a change in the total concentration ofimpurities in the sample gas.

U.S. Pat. No. 6,924,479 issued to Blanchard Aug. 2, 2005 is directed toion injection in a drift tube apparatus for mobility spectrometrywithout conventional ion shutters such as the Bradbury-Nielson orsimilar designs common to such drift tubes. Instead ions were passedbetween the ion source and drift region by using time-dependent electricfield gradients that act as ion barriers between ordinary drift rings.Benefits of this design are simplicity and mechanical robustness. Thision injection technique dynamically accumulates the ions prior to theirrelease into the drift region of the apparatus instead of destroying theions created between shutter grid pulses, as does the Bradbury-Nielsonmethod. The invention provides not only structural improvements to thewell known drift tube apparatus, but also claims to provide inventivemethods for operating a drift tube apparatus to achieve maximum analyteinjection efficiency and improving ion detection sensitivity. Improvingion detection sensitivity of drift tubes has practical experimentalapplication. Incorporation of the inventive apparatus into a smokedetector is a further practical application of the invention.

U.S. Pat. No. 6,828,795 issued to Krasnobaev et al on Dec. 7, 2004, isdirected toward an explosive detection system which detects the presenceof trace molecules in air. The sensitivity of such instruments isdependent on the concentration of target gas in the sample. The samplingefficiency can be greatly improved when the target object is warmed,even by only a few degrees. A directed emission of photons, typicallyinfrared or visible light, can be used to significantly enhance vaporemission. The sensitivity of such instruments is also dependent on themethod of gas sampling utilized. A cyclone sampling nozzle can greatlyimprove the sampling efficiency, particularly when the sampling needs tobe performed at a distance from the air intake.

What is lacking in the prior art is a teaching of a combination ofcomponents which act in concert to provide a miniaturized handheld IMSdevice having enhanced sensitivity and performance for the detection oftrace chemicals. Thus, if a highly sensitive and miniaturized IonMobility Spectrometer (IMS) could be produced, with demonstratedperformance at elevated temperatures, a long felt need in the art wouldbe met.

SUMMARY OF THE INVENTION

By utilizing the combination of a unique electronic ion injectioncontrol circuit in conjunction with a particularly designed drift cellconstruction, the instantly disclosed ion mobility spectrometer achievesincreased levels of sensitivity, while achieving significant reductionsin size and weight. The instant IMS is of a much simpler and easy tomanufacture design, rugged and hermetically sealed, capable of operationat high temperatures to at least 250° C., and is uniquely sensitive,particularly to explosive chemicals.

A unique ion reservoir is achieved in which ions are temporarilycollected prior to injection into the drift region of the IMS. Thisfeature increases the sampled ion population allowing more time forreactions between the reactant ions and sample molecules thus increasingthe signal-to-noise parameter as well as over-all sensitivity to a givenconcentration of sample chemicals. This unique feature allows for bettersensitivity while permitting smaller design geometries producing arelatively small device.

In order to achieve an efficient ion reservoir, an innovative electronicion injection control circuit is provided that is much simpler thancurrent designs for IMS. This circuit operates off of a low voltagetrigger timing pulse which trips an opto-isolator. This device is partof an innovative resistive bridge circuit connected to a high voltagetransistor. The trigger pulse to the opto-isolator causes the voltage tothe base of the high voltage transistor to vary with the pulse. Thisallows the transistor to provide a sharp square wave voltage pulse tothe ion control ring. The resultant large drop in voltage from the pulsecauses the ions in the ion reservoir to be injected into the IMS driftregion. Between pulses, the ion control ring is in a high voltagecondition which stops the ions in the ion reservoir. This circuit, in avery simple and reliable way, enables the high voltage switching(typically between 800 and 1000 volts) to be accomplished, which permitsthe establishment of the ion reservoir described above.

The above described enhancements produce an added benefit in the form ofa gridless IMS design. While the majority of current IMS designs rely onthe use of ion control and screen grids to provide a uniform controlvoltage radially across the ion drift tube; the ion injection circuit asdescribed above operates at sufficiently high voltages such that the useof these grids was found to be unnecessary in the unique IMS drift cellconstruction subsequently described. Effectuating an embodiment whichdoes not require a complicated grid design greatly simplifies theconstruction of the IMS and also virtually eliminates microphonic noisepickup. The ion injection circuit above can be thought of as using a“virtual” grid to control the ion movement.

Additionally, a unique IMS drift cell construction is herein providedwhich employs a hermetic construction using ceramic insulating ringsjoined to a nickel cobalt ferrous alloy (such as Kovar®) metal rings byan “active metal” joining process. This ceramic-metal design allows thecell construction itself to be its own enclosure. In prior designs, theIMS drift cell structure was enclosed in an outer housing to isolate itfrom the operating environment. Since the cell is operated at highvoltages, somewhat complicated means had to be provided to electricallyinsulate the cell from the enclosure.

Furthermore, complexities arose in providing high voltage connections tothe cell through the enclosure, and to make the signal connections. Inthe instant design, the metal rings are manufactured with tabs thatconnect directly to the high voltage control and electrometer board. Thehermetic design of the drift tube allows this unique IMS to be used forapplications requiring that no outside contaminants be introduced, suchas for the analysis of ultra high purity gases. Also, by virtue of theactive metal joining process which requires the cell structure to befired at temperature near 1000° C., all contaminants in the cellstructure having any measurable vapor pressures are removed, so that innormal operation the cell does not outgas, and can be stored for lengthyperiod of time without buildup of contaminants from the slow outgassingof materials as is a problem in many current IMS designs. This novelcell can also be operated at much higher temperatures than current IMSs.

A special cell enclosure which provides for heating the cell, insulatingthe heated cell from other instrument components, and isolating the cellfrom spurious electronic signals and interferences is provided to enablethe actual mounting and operation of the hermetic drift cell. A thinfoil heater was designed to wrap around and heat the cell. The heater isa Kaptan® high temperature plastic sandwich which is electricallyinsulated when in contact with the cell high voltage rings and does notaffect the electrical operation of the cell. This is a novel applicationof this kind of heater for an IMS cell, and is made possible by thesimplified design of the cell itself. Additionally, the heater iscontrolled using a pulse-width-modulated voltage supply operating at ahigh frequency so that there are no heater pulses or relay pulses toperturb the IMS signal. Heater pulses are a significant contribution tonoise in the spectra of conventional IMS devices. The cell and heaterare encased in a special lightweight insulating material which then iscontained in a plastic housing. The housing is either coated with aspecial resistive paint or impregnated with metal so that the housingfunctions as an electric field shield isolating the cell from outsidespurious electrical signal and interferences.

Since the ion reservoir concept allows the concentration of ions andgreater ion sampling efficiencies over the standard IMS design, a lowlevel Americium 241 ionization source can be utilized, e.g. as low as 1microcurie. This has a similar strength as the Am-241 sources employedin commercial smoke detectors, greatly simplifying or eliminatingregulatory requirements. However, since the IMS cell requires hightemperatures for manufacture, it is not appropriate to do this with theradioactive source installed. Also, the IMS cell may be manufactured atunlicensed facilities, so that the presence of radioactive sources isnot permitted at the manufacturing site. For these reasons a uniquesource design and installation procedure was devised which allows thesource to be easily installed at a licensed facility, after the IMS cellbody has been made.

A specially coated gas inlet for the IMS was designed which allows forthe very efficient inhalation of certain chemicals (specificallyexplosive molecules and particles). Explosive molecules are by theirnature fragile and heat labile. They are also extremely “sticky”, sothat a delicate compromise has to be determined balancing gas flow ratesand the surface temperatures to which the explosive molecules aresubjected. The inlet piece is separately heated by the same thin foilheater used to heat the cell. An insulating sleeve made of the samematerial as used to insulate the cell fits around the heated gas inlet.This inlet configuration is then enclosed in a unique sampling nozzledesign, made from a special relatively inert high temperature plastic.Gas ports in the nozzle blow gas at the surface to be sampled atcarefully determined angles so that explosives can be efficientlysampled from surfaces. The inhalation inlet allows these trace explosiveresidues to be effectively introduced into the detector housing of theIMS device for measurement. A unique, single pump gas flow design isemployed to both blow air through the nozzle ports, inhale the sampledgas into the IMS inlet, and to provide drift gas low for the IMS.

Accordingly, it is a primary objective of the instant invention toprovide an ion mobility spectrometer which achieves increased levels ofsensitivity, while achieving significant reductions in size and weight.The instant IMS is of a much simpler and easy to manufacture design,that is rugged and hermetically sealed, capable of high temperatureoperation to at least 250° C., and is uniquely sensitive.

It is a further objective of the instant invention to provide an ionmobility spectrometer which incorporates an ion reservoir for providingenhanced sensitivity.

It is yet another objective of the instant invention to teach anelectronic ion injection control circuit which enables high voltageswitching in a manner which permits establishment of an ion reservoir.

It is a still further objective of the invention to provide an ionmobility spectrometer which is gridless.

It is still an additional objective of the instant invention to providea drift cell construction for an ion mobility spectrometer whicheliminates the introduction of outside contaminants and precludes theformation of outgassed contaminants internally.

Yet another objective of the instant invention is the provision of aspecial cell enclosure for mounting and operation of the hermetic driftcell.

An additional objective of the instant invention is the provision of alow-level ionization source, e.g. AM-241, along with a uniqueinstallation procedure.

Yet an additional objective of the instant invention is the provision ofa unique sampling nozzle design, which allows for extremely efficientinhalation of contaminants.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with any accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention. Any drawings contained hereinconstitute a part of this specification and include exemplaryembodiments of the present invention and illustrate various objects andfeatures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will befurther appreciated, as they become better understood by reference tothe detailed description when considered in connection with theaccompanying drawings:

FIG. 1. Schematic Diagram of Standard Prior Art IMS Function;

FIG. 2. Typical Ion Control Circuit Design;

FIG. 3. Design Schematic of Explosives Detection Drift Cell;

FIG. 4. Guard Ring;

FIG. 5. Pure Gas Analysis Prototype Drift Tube;

FIG. 6. Heater Schematic;

FIG. 7. Cell Enclosure Assembly Exploded View;

FIG. 8A relates to a drift cell assembly, and includes an expanded viewof the source holder, as it communicates with the drift cell, as well asthe collector assembly; FIG. 8B is an assembled view of the sourceholder; FIG. 8C is a side view of the drift cell collector assembly, andFIG. 8D is a detailed view of the collector assembly hole configuration;

FIG. 9. Source Holder;

FIG. 10. Source Assembly;

FIG. 11. Source Fixture Assembly;

FIG. 12. Source Ceramic Isolator;

FIG. 13. Explosives Detection Drift Cell Inlet;

FIGS. 14A-E are directed toward the Sample Detector Nozzle, and showdetails of the nozzle gas port (14A), details of the nozzle design(14B), the nozzle insulator (14C), a perspective exterior view of thenozzle (14D) is shown, as well as a perspective interior view (14E)thereof;

FIG. 15 illustrates a Negative Ion Spectrum;

FIG. 16 is a cross-sectional view of a hand-held ion mobilityspectrometer;

DETAILED DESCRIPTION OF THE INVENTION

Ion Mobility Spectrometry General Description, Operation, and Theory:

The IMS, while complex in many of its aspects is conceptually easy todescribe. In general terms, it can be thought of as an electronic, gasphase, atmospheric pressure, trace chemical analyzer providing lowpicogram sensitivity for many chemicals with chemical discriminationbased upon ion mobility. Structurally, it is simply an electric fielddrift tube with an ionizing source, a means for injecting the ions intothe drift tube, and an ion collector that electrically measures theions. In more detail, it is most easy to describe the structure andoperation of a standard prior art IMS using a schematic diagram, as setforth in FIG. 1.

In the standard model of the IMS, the drift tube consists of a series ofstacked cylindrical rings insulated from each other and ground. Therings are connected to each other in series to a resistive voltagedivider, which when a high voltage is supplied, energizes eachsuccessive ring in the stack at a uniform progressively lower voltageestablishing a uniform linear field gradient along the axis of the drifttube. In some instances, instead of stacked rings, a one piece resistivecoated tube has been used, but because of the high resistance required,in practice, these are difficult to manufacture consistently. The highvoltage is applied to the source end of the cell; the collector end isnear ground potential.

Normally, the entire cell is at or near atmospheric pressure. Samplevapors enter the ion molecule reaction region as shown in FIG. 1. Inalmost all configurations of a standard IMS, a radioactive nickel-63ionization source is used. The typical activity is 15 millicuries.Radioactive sources using tritium or americium-241 have been used, aswell as other kinds of ionization mechanisms, such as UV, electricaldischarge and coronas, x-rays, and high energy lasers. Ionization of thecarrier gas containing the sample vapors results in the formation ofcertain reactant ions depending upon the nature of the gas. For example,in ambient air the positive ions formed are initially molecular nitrogenand oxygen ions which quickly transfer charge to the water moleculespresent in ambient air to produce water cluster ions. The negative ionsformed are primarily molecular oxygen ions. Ions of both signs arecontinuously produced in the neutral plasma near the radioactive source,which in the absence of an electric field quickly recombine to theirneutral state. Once voltage is applied, the negative and positive ionsare separated. If positive voltage is applied, the negative ions arepulled into the source ring at high voltage, while the positive ions arerepelled into the drift tube where they follow the electric fieldgradient toward the collector end of the drift tube. Applying negativevoltage reverses the situation, and negative ions move down the drifttube. The positive water cluster ions and negative oxygen ions arecalled the reactant ions, because they readily and very efficientlyreact with most trace constituents in the gas to produce the productions. In the figure, the reactant ions are identified as R+, and thesample molecules as A, B, and C, which become the product ions A+, B+,and C+.

Many ion molecule reactions are very fast and are the reason for thehigh sensitivity of the IMS technique. Concentrations as low as 10-14have been measured using IMS. From a practical standpoint, tracechemical levels in the low picograms (10-12 gm) or lower can bemeasured. Another important consideration is that the response isproportional to the reaction time. This has usually been taken to be thedrift time of the reactant ion in the reaction region, and is typicallyjust a few milliseconds.

All these ions thus formed move through the reaction region toward theshutter grid. In a standard IMS the shutter grid is of theBradbury-Nielsen type, which consists of interdigitated wires formedacross a ring. Adjacent wires are insulated from each other and normallyhave small opposite voltage biases applied. If, for example, the shuttergrid is at an average potential of 2000V, the two sets of wires may beat 1985V and 2015V, having a bias voltage of ±15V applied relative tothe shutter grid average voltage. Since the wires are closely spaced,relatively high electric fields are established near the wires radiallyto the main drift tube field gradient. Ions moving to this grid arerapidly attracted to one set of wires and discharged. Thus the grid inits “closed” condition stops all ions from moving farther into the drifttube. To “open” the grid, the voltage biases to the wires are removed,and the ions follow the drift tube field gradient into the drift region.There is some ion loss due to ions hitting the wires, but very finewires are used, typically providing an optical transparency of 80% ormore to the grid. The grid is then repetitively opened for shortintervals admitting pulses of the mixed ions into the drift region. Thefunctioning of the shutter grid operates on millisecond time frames.Typically, the shutter grid is opened every 20 to 50 msec for about 0.2to 0.5 msec. Needless to say, the construction of the shutter grid andthe required control circuitry is very complex, and has beenhistorically the most difficult to manufacture and costly component inion mobility spectrometers. Additionally the fine, taught wires used aremicrophonically sensitive, so that the shutter grid operates as anacoustic microphone, contributing noise to the ion mobility spectra fromany outside vibrations.

Returning to the figure, as the mixed pulse of ions move down the drifttube they separate into their different chemical ion species bases upontheir differing mobilities in the drift medium, typically air. Usuallythe air in the drift region is moving in a counter-current direction tothe ion flow. The drift gas generally does not contain the samplemolecules, which effectively quenches the ion-molecule reactions thatoccur in the reaction region. This allows for the clean separation ofpeaks in the drift region. The individual ion species can be thought tochromatographically separate in the “stationary phase” of the driftmedium. Thus the early name for IMS, plasma chromatography, was usedbecause a plasma was formed which was then separatedchromatographically. Whereas a gas or liquid chromatograph typicallyproduces chromatograms in minutes, the “plasma chromatograph” produced“chromatograms in milliseconds. The name ion mobility spectrometry wastaken up early on to indicate many of the similarities between IMS andmass spectrometry. The IMS can be thought of as being a time-of-flightmass spectrometer operated at atmospheric pressure. The IMS does nothave the resolution of a mass spectrometer, but it is much less complexand in many applications actually has greater sensitivity.

Molecular weight and ionic cross section (shape) both affect mobility.Under the applied field the ions undergo thousands of collisions withthe air molecules they encounter, which causes the ions to move at anaverage terminal velocity as opposed to continuously accelerating in theelectric field. Within each pulse of ions, diffusion processes cause theion pulse to acquire a bell curve or semi-gaussian shape. The arrival ofthe individual pulses at the collector electrode produces acharacteristic ion arrival time spectrum as shown in the figure. Thecollector is almost always of the Faraday type, and is connected to afast electrometer which converts the very small ion currents to sensiblevoltages which can then be read out on an oscilloscope to view thespectra. Modern data processing techniques allow the rapid recording andviewing of the spectra. Since the spectra are generated so quickly,typically a number of spectra are accumulated and signal-averaged toimprove signal-to-noise. Fifty 20 msec spectra, for example produce anaveraged spectrum every one second or so, which is usually more thansufficient for most applications.

Operation and Advantage of Ion Reservoir Design:

In the instantly disclosed miniaturized IMS, a different approach hasbeen taken to produce and inject the ions into the drift region. Theshutter grid has been eliminated, but the equivalent ring does the samefunction by having a relatively high voltage applied to it, thusreversing the field at this point to stop the ions. The level of fieldreversal has to be quite high to accomplish this. The effect of doingthis is to create an ion reservoir in the space above the high potentialring. To take an example; if 1000V is applied to a drift tube 10 cm longwith rings spaced at 1 cm intervals, then the voltage difference betweeneach ring would be 100V. The source ring would be at 1000V, the nextring down would be at 900V, the next at 800V, and so on to produce auniform field gradient down the tube. If then, say the 5th ring downinstead of being at 600V were set to 800V, then the sequence of ringvoltages down the cell would be: 1000V, 900V, 800V, 700V, 800V, 500V,400V, etc. The 4th ring at 700V now is at low potential relative to therings above and below it at 800V. The ions moving down the tube from thesource would stop at this low potential area. However, the source iscontinuously producing ions which march down the drift tube until thelow potential area is encountered, where they “pile up” on top of theions already there. Thus, the ions accumulate in this low potentialreservoir. The population of ions in the reservoir is dynamic in thatthe low of incoming ions is eventually balanced by the loss of ions tothe walls of the drift tube primarily through diffusion. If the ionconcentration becomes high enough, space charge and mutual repulsioneffects can also limit the ion concentration. Diffusion can be shown tooperate on time frames multiple 10s of milliseconds, so that if the ionpopulation is sampled every 20 msec or so, most of the accumulated ionsare still present in the reservoir. The reservoir is sampled andinjected into the drift region by reverting the higher potential 5thring from 800V to 600V which then reestablishes the uniform fieldgradient down the drift tube. The mixed ions in the reservoir then moveinto the drift region and separate into individual ion peaks aspreviously described for the standard IMS. The cell voltage remainsuniform for a sufficient time to allow the all the ions in the reservoirto move past the 5th ring into the drift region of the cell. In practicethis time is 1 to 5 msec. Then the higher potential is reapplied to the5th ring and the process repeats. Typically, the reservoir is sampledevery 20 msec, which gives enough time for all the ion peaks to bemeasured.

The ion reservoir technique has a number of important consequences apartfrom providing an alternative to the standard shutter grid technique. Inthe standard IMS the shutter grid is only opened for typically 1% of thetime. This means that only 1% of the ions being generated at the sourceare actually sampled. 99% are lost. Also, the reaction time for theproduction of sample ions is only the relatively short time it takes areactant ion to traverse the length of the reaction region. With the ionreservoir, proportionally a much greater number of ions are available tobe measured. Theoretically, this could be 100%, but in actuality lossesin the reservoir reduce this number. Additionally, as the ions reside inthe reservoir between sampling intervals, the reactant ion has more timeto react with the sample molecules. Both of these effects increase theinherent sensitivity of this technique to any given level of samplechemicals over the standard model.

The increase in the number of ions sampled usually does not lead to agreat increase in the sensitivity of IMS due to two factors: thesignal-to-noise improvement is not usually proportional to the increasednumber of ions sampled, and for the relatively energetic nickel-63sources used, the space charge repulsive effects can limit the abilityto appreciably concentrate ions in the reservoir. However, thecapability of utilizing the ion reservoir does allow the use of a lessenergetic source. In prototype work a 0.9 microcurie americium-241source was used with excellent results. In the instantly disclosedminiaturized IMS, for mechanical reasons, a 20 microcurie americium-241source is used. The peak intensities observed using this source with theion reservoir are equivalent or better than observed in a standard IMSusing 15 millicuries of nickel-63 having almost 1000 times the activity.

Also, because of the longer reaction times available, the length of thereaction region can be greatly reduced, and the entire drift tubeminiaturized. The volume of the instant IMS is about 5% that of astandard IMS, with a concomitant reduction in weight, as well. Smallervolumes have practical measurement advantages, because less gas is usedto operate and clear the drift tube. The use of the ion reservoir alsogreatly simplifies the control electronics.

Description of Electronic Ion Injection Control Circuit:

The electronic ion injection control circuit useful in the instantlydisclosed IMS is illustrated in FIG. 2. The functioning of this circuitis controlled by a low voltage trigger timing pulse as shown in thefigure. This trigger pulse can be generated externally in a variety ofways, such as from a trigger circuit or from a computer generated pulse.The trigger pulse should have the form of a square wave with anamplitude of +5V to +15V, a repetition period of typically 10-50 msec,and a pulse width of 1-20 msec. Although, in order to generate properpeaks in the IMS from the ion reservoir, the pulse width required hasbeen experimentally shown to be from 1 to 5 msec in duration.

The organization of a generic ion control circuit of our design can beseen in FIG. 2. In this particular circuit, high voltage is suppliedfrom a +1500V power supply. There are three legs coming from the +1500Vpower supply. Each leg has its own path to the three grounds shown. Thelegs marked with +903 volts and +1100 volts voltage labels are separatedue to the action of Q2, a high voltage PNP transistor, and the diode,D1. Because the leg with the italicized voltage values only contacts the+1100 volt leg at the base of Q2, this also has an independent ground.Considering first the situation when Q1 and Q2 are both open, then the+1100V, italicized and +903 voltage labels apply. In this condition, thevoltage paths are relatively simple. The left hand +100V leg has a totalresistance value of 15 Megohms. The center italicized leg has aresistance value of 16 Megohms. The right hand +903V leg has a totalresistance of 15.07 Megohms, due to the effect of the 200 Megohmresistor and the 9.5 Megohm resistor being in parallel when Q2 is open.The calculated voltages at various points in the circuit are shown onthe figure. For instance, the 4 Megohm resistor at the top of the lefthand leg coming down from the +1500V power supply then provides a 400V(4÷15=0.26667,×1500V=400V) voltage drop, setting the voltage below thisresistor at +1100V, as shown. This is the voltage supplied to theemitter of the transistor Q2. Since, in this condition, the base of Q2is at +1125, and is greater than the emitter voltage of +1100 V, Q2 isin an open condition (base more positive). With Q2 open, the voltage tothe control grid through the diode D1 is calculated to be +903V. This isthe open position for the ion control ring, allowing ions to pass intothe drift region of the cell, This condition holds during the 1 to 5msec trigger pulse to Q1.

Q1 is a high voltage opto-isolator. The action of the low voltagetrigger pulse opens or closes Q1 to the passage of current. When Q1 isclosed, then there is a parallel resistance path formed on theitalicized leg. There are now 5 Megohms in parallel with 5.121 Megohms(neglecting the resistance of Q1). This yields an equivalent resistancefor the two branches together of 2.53 Megohms. Thus, the totalresistance of the italicized leg drops from 16 Megohms to: 4 Megs+2.53Megs+7 Megs=13.53 Megohms. Now, the 4 Megohm resistor at the top of thisleg provides a greater percentage voltage drop from the +1500V supplywith the total resistance reduced to 13.53 Megohms. This voltage drop is444V which sets the voltage below this 4 Megohm resistor at +1056V asindicated by the bold voltage values. Since the voltage supplied to thebase of Q2 in this condition of +1056 V is now less than the emittervoltage of +1100V, Q2 closes, which provides an alternate voltage pathto the ion control ring. The 200 Megolun resistor is now in parallelwith the 11 Megohm resistor on the left hand leg, which sets the voltageto the ion control grid at +1084V, as shown. The diode D1 preventscommunication to the +903V of this leg isolating this voltage from theion control ring and permitting proper functioning of the circuit. Thecontrol grid voltage varies between 903V (open to ions) and 1084V(closed to ions) with the operation of the trigger pulse.

Thus, as described before, this circuit, by means of the actions of thelow voltage trigger timing pulse, the high voltage opto-isolator, andthe resistive bridge circuit connected to a high voltage transistor,allows this transistor to provide a sharp square wave voltage pulse tothe ion control ring. The large drop in voltage from the pulse causesthe ions in the ion reservoir to be injected into the IMS drift region.Between pulses, the ion control ring is in a high voltage conditionwhich stops the ions in the ion reservoir. This circuit, in a verysimple and reliable way, enables the high voltage switching (in thisexample between +903V and 1084V) to be accomplished, which effectivelypermits the establishment of an ion reservoir; which heretofore was notpossible following the teachings of the prior art, e.g. Blanchard's U.S.Pat. No. 4,855,595.

The 121 K ohm resistor in series with Q1 is used to protect it from highcurrents, allowing a voltage drop of only 7V. The current through Q1calculates as 1500V+13.53 Megohms=110 microamps. The current through theclosed Q2 and down the 200 Megs to ground is only 5.7 microamps. Theselow currents also enable miniaturized surface-mount-technology, SMT,components to be used greatly lowering the cost of producing thiscircuit. Q1 cannot be used by itself to set the voltages to the ioncontrol ring, because this component alone cannot provide the properhigh voltage pulse peak shape. It also may be that because of therelatively large voltage differences required, the voltage and currentlimits the miniature opto-isolators are close to being exceeded. Theopto-isolator provides the few microseconds of quiescent state that thebase of the transistors need for settling time. It also aids inextending the life of the switching transistors, as well as providingisolation between the high voltage and low voltage trigger pulse. Thecircuit as described here permits only a small voltage change of 69V tothe base of the transistor to accomplish a 181V voltage change to ioncontrol ring. The circuit described can be easily switched to a negativevoltage circuit by changing the transistor from PNP to NPN, andreversing the direction of the diode D1.

All of the resistor values and voltages used can be changed somewhat tovary the voltages supplied to the ion control grid to optimizeperformance in any given analysis application, but the essentials ofthis circuit remain unchanged. In addition, a 1 Megohm resistor can beadded in series with the 200 Megohm resistor shown in FIG. 2 to providea low voltage test point to monitor the actual voltage characteristicsof the ion control ring.

Various voltages have been used for the positive ion circuits dependingupon the application. For instance, for the NASA project operating inhelium, the ion control and voltage divider circuits were run at +345 Vand +280 V respectively. Due to the low breakdown potential of helium,lower voltages are required in order to eliminate possible arcing. Thecircuits performed fine over these ranges of voltages.

IMS Drift Cell Construction:

The ion detection assembly includes a unique IMS drift cell constructionwhich employs a hermetic construction using ceramic insulating ringsjoined to Kovar® metal rings by an “active metal” joining process. Thisprocess is commercially available from various companies, and theparticular process used for the fabrication of prototypes wasproprietary to the fabricator. This ceramic-metal design allows the cellconstruction itself to be its own enclosure. FIG. 3 shows the aprototype design structure or the explosives detection drift cell. Theactive metal brazing process achieves a permanent bond between a propermetal such as Kovar® and a high grade alumina based ceramic, using afiring fixture. The process is relatively quick and simple. A number ofdesign innovations were incorporated into the drift cell constructionfor both performance improvements and to simplify fabrication. It waslearned that the Kovar® rings could be stamped to include a tab formaking the electrical connection. The design of this is shown in FIG. 4.This piece was quite thin with a thickness of only 0.031″. This hadthree advantages. The thin Kovar® pieces are less costly to produce, putless strain on the ceramic during the joining process, and shouldprovide a more uniform field in the IMS cell. Thicker metal guard ringsproduce larger “steps” in the field gradient. Thin guard rings produce amore uniform field. Having the tab stamped as part of the ring greatlysimplified assembly of the cell since it was no longer required toeither weld the tabs onto the rings or to spot weld the connectingwires. The tabs are designed to be plugged directly into a circuit boardcontaining the resistive bleeder string.

It was determined that the preferred procedure is to use Kovar® ringsand ceramic spacers with the same outer dimensions. This allows a simplefixture to be used to build the cell. The diameters of the rings andspacers could now be the same because the tabs on the rings allow theelectrical connections to be easily made. This also makes engineeringthe heater design very easy because the cell can be easily placed in aninsulated heater block which contacts all the surfaces of the cell forproper heating. The ceramic inner wall is at a distance from the inneredge of the metal guard ring which should also reduce any static effectson the voltage gradient field.

The ceramic and Kovar® parts are assembled vertically, piece by piece,in a firing fixture. The fixture containing the yet unbrazed assembly isthen placed in a furnace at approximately 1000° C. to complete theactive metal brazing process. FIG. 5 shows a version of the drift cellwhich can be used for pure gas analysis. The end caps and side tubesrequired careful considerations in their design to prevent fractures andcontamination during the firing process. The end tube pieces weredesigned so that they had a shallow cap which then could fit over a stepmachined on the end of the mating ceramic end piece. In this way thebrazing was done on the diameter rather than the flat.’ This producescompressive stresses during firing or cooling, which the ceramic easilytolerates.

As can be seen in the explosives detection drift cell drawing shown inFIG. 3, the side tube was designed to it over a boss formed on theappropriate ceramic spacer. This was either the center spacer as shownin FIG. 5, or also included the spacer near the inlet as shown in theFIG. 3 drawing. This boss had a step at the end so that a Kovar® stubcould fit over it, similarly as the end tubes. As shown in FIG. 5, a ⅛inch stainless steel tube was then welded to the Kovar® stub. Thisdesign provides a rugged construction with the brazing done on therelatively large outside diameter of the ceramic boss.

The end caps were of two designs. One type is shown in FIG. 5. The endcaps have ¼ inch stainless steel tubes welded on to them, to which gastight (typically VCR) fittings can be welded in order to make acompletely gas tight cell for analysis of high purity gases. The othertype of end caps are shown in the explosives detection drift celldrawing (FIG. 3). These caps are tapped for ⅛ inch Swagelok® fittingthreads. A ⅛ inch Swagelok® union elbow could then be threaded into therear end cap, and the inlet fitting threaded into the inlet cap.

Another very important feature of the ceramic-metal design is the use ofvery high resistance ceramic components. The electrometer and iondetection circuit of this IMS is exceedingly sensitive, being capable ofmeasuring femtoamps. The cell is operated at high voltages, so that verysmall leakage currents through the cell insulators can be a greatproblem. It has been calculated that the ceramic insulators need toprovide 10,000 megohms of resistance for best performance.

In prior designs of the ion detection assembly, the IMS drift cellstructure was enclosed in an outer housing to isolate it from theoperating environment. Since the cell is operated at high voltages,somewhat complicated means had to be provided to electrically insulatethe cell from the enclosure. Also further complexities arose inproviding high voltage connections to the cell through the enclosure,and to make the signal connections. IMS cells are normally difficult tomanufacture due to their complexity and the stringent electrical andcleanliness requirements of the technique. The prior art fails to teacha design that is as inherently simple, rugged, and clean as the hereindisclosed design. This ceramic-metal design allows the cell constructionitself to be its own enclosure. The hermetic design of the drift tubeallows this unique IMS to be used for applications requiring that nooutside contaminants be introduced, such as for the analysis of ultrahigh purity gases. Also, by virtue of the active metal joining processwhich requires the cell structure to be fired at temperature near 1000°C., all contaminants in the cell structure having any measurable vaporpressures are removed, so that in normal operation the cell does notoutgas, and can be stored for lengthy period of time without buildup ofcontaminants from the slow outgassing of materials as is a problem inmany current IMS designs. This novel cell can also be operated at muchhigher temperatures than current IMSs.

Mounting and Operation of the Hermetic Drift Cell:

The actual mounting and operation of the hermetic drift cell makes useof a special cell enclosure which provides for heating the cell,insulating the heated cell from other instrument components, andisolating the cell from spurious electronic signals and interferences.Again, the enclosure provides superior performance at a very low weight,just a few grams.

A thin foil heater was designed to wrap around and heat the cell. Theheater is a Kaptan® high temperature polyimide plastic sandwich which isinsulated itself electrically, from the cell high voltage rings and doesnot affect the electrical operation of the cell. The particular heaterused in the prototype development work made was by Thermal Circuits Inc.of Salem, Mass. A schematic of this heater is shown in FIG. 6. Theheater elements cover virtually the entire surface of the heater. Thisthin foil heater has two separate heater zones which allow the driftcell body and inlet to be independently heated. The heater zones aredesigned so that more power is supplied to the inlet, where heat lossesare higher. As can be seen in the drawing, this heater was composed oftwo decks, one for the heater elements, and one for the RTD temperaturesensors. The temperature sensors consist of 100 ohm platinum RTDs. Thisis a novel application of this kind of heater for an IMS cell, and ismade possible by the simplified design of the cell itself. Additionally,the heater is controlled using a pulse-width-modulated (PWM) voltagesupply operating at a high frequency so that there are no heater pulsesor relay pulses to perturb the IMS signal. Heater pulses are asignificant contribution to noise in the spectra of conventional IMSdevices. The heater control is handled by a microprocessor which readsthe RTDs and sets the PWM frequencies to provide the correcttemperatures. The thin foil heaters are flexible and light weight, andeasily wrap around the cell and install into the insulator block.

The cell and heater are encased in a special lightweight insulatingmaterial which then is contained in a plastic housing. The housing iseither coated with a special resistive paint or impregnated with metalso that the housing functions as an electric-field shield, isolating thecell from outside spurious electrical signals and interferences. Theinsulating material is ZIRCAL-18 Refractory Board, manufactured byZircar Refractory Composites, Inc. of Florida, N.Y. This is a hightemperature calcium silicate block insulation with excellent mechanicalproperties that combines relatively high strength and excellent thermalinsulating characteristics. At 200° C. the ZIRCAL-18 has about twice thethermal conductivity of still air.

When used with the heated Mini-Cell, very satisfactory results wereobtained. The outside of the insulator block approaches 45° C., when thecell was operated at 200° C., but this is still very acceptable. Onefurther advantage of the ZIRCAL-18 is that it is a relatively strongmaterial that is easily machined.

FIG. 7 shows the assembly of the cell enclosure inclusive of the driftcell 10, insulator housing 79A having exit port holes 73, and insulatorhousing 79B for receipt of fastening screws 78 therethrough. To thedrift cell 10 are mounted threaded receiver gas connector 71, to whichSwagelok fitting 75 is coupled. On its opposite end rift cell 10 is influid engagement with stainless steel sample inlet 72, which isthreadably connected to receiver 74. Sample nozzle 80 is in turn fluidlycoupled to sample inlet screw 72. Detector insulators 76A and 76B aresandwiched about drift cell 10 and encased by housings 79A and 79B uponassembly.

Ionization Source Design and Installation:

Since the ion reservoir concept allows the concentration of ions andgreater ion sampling efficiencies over the standard IMS design, a lowlevel Americium-241 ionization source 83 (see FIG. 8A) could be used.This has a similar strength as the Am-241 sources employed in commercialsmoke detectors, which greatly simplifies or eliminates regulatoryrequirements for the instant IMS. However, since the IMS cell requireshigh temperatures for manufacture, it is not appropriate to do this withthe radioactive source installed. Also, the IMS cell may be manufacturedat unlicensed facilities, so that the presence of radioactive sourcesare not permitted at the manufacturing site. For these reasons a uniquesource design and installation procedure was devised which allows thesource to be easily installed at a licensed facility, after the IMS cellbody has been made.

FIG. 8A relates to a drift cell assembly, and includes an expanded viewinlet assembly 81, and source holder 82, as it communicates with thedrift cell 10, the drift cell having exit ports 85, as well as thecollector assembly 88. The expanded view of inlet assembly 81illustrates the radioactive source 83, positioned within source holder82, in the explosive detection drift cell 10. FIG. 8B shows an assembledview of inlet assembly 81. FIG. 8C is a sideview of drift cell collectorassembly 88, and FIG. 8D further shows the detail of the holeconfiguration 87 of the collector assembly, which prevents direct accessto the source from the rear end of the drift cell assembly 86. There area number of components relative to the installation of the source asshown by the subsequent drawings. FIG. 9 shows the source holder, whichis a standard 10-32 stainless steel socket head screw with a cup pointthat has been drilled out as shown in the drawing. These are sent to apurveyor of radiation materials and ionization sources, e.g. NRD LLC,which installs a 20 microcurie americium-241 foil into the source holderas shown in FIG. 10.

FIGS. 11 and 12 show the source fixture assembly and source ceramicisolator which are located in the IMS drift tube shown in FIG. 8. Usingan appropriately sized standard hex tool, the source holder is easilyinstalled into the source fixture, after the cell body has been fired.This design and installation procedure is completely unique and allowsthe manufacture of the ceramic-Kovar® IMS cell without the need toconsider the radioactivity at the manufacturing location.

Sampling Nozzle Design:

A specially coated gas inlet for the IMS was designed which allows forthe very efficient inhalation of certain chemicals (specificallyexplosive molecules and particles). Explosive molecules are by theirnature fragile and heat labile. They are also extremely “sticky”, sothat a delicate compromise has to be determined balancing gas flow ratesand the surface temperatures and composition to which the explosivemolecules are subjected. FIG. 13 shows the inlet piece that threads intothe end cap of the explosives detection cell as shown in previousfigures. This piece is subjected to a proprietary process of RestekCorporation called Silcosteel® treatment which inactivates the stainlesssteel surface of the inlet piece. Using the thin foil heater previouslydescribed, the inlet is normally operated at 150° C. to 180° C. forexplosives detection. Without the Silcosteel® treatment much of theexplosive material would be catalytically destroyed contacting thesurface as the ambient air containing the explosive is inhaled into theinstrument. The ID of the inlet piece is only about ⅛ inch whichprovides a relatively high velocity to the inhaled gas flow, reducingcontact of the explosive material on the surface. The inlet piece iscontained in a thin tube of the same ZIRCAL-18 refractory board materialused to insulate the IMS cell, as discussed previously.

These two pieces together fit into a unique nozzle, the particulars ofwhich are described in FIGS. 14A-14E. This nozzle is made from PEEK™, arelatively inert high temperature plastic. Exhaust gas ports in thenozzle blow gas at the surface to be sampled at carefully determinedangles so that explosives can be efficiently sampled from surfaces. Asshown in FIG. 14A, the three ports are angled at 25° from the axis ofthe nozzle. As shown in FIG. 14B, each exhaust gas port iscircumferentially disposed at 120° spacings on the interior edge of thenozzle. FIG. 14C is a section view of the nozzle through line A-A aswell as a top view along the inlet nozzle axis.

The directed air flow from the three ports converge approximately 1 inchin front of the nozzle. Material on a surface is efficiently removed atthis point via surface perturbation, and directed through the gas inletnozzle along the longitudinal axis thereof. In addition, the interiorsurface of the nozzle is slightly concavely curved, which aids in thesample introduction. This inhalation inlet assembly allows traceexplosive residues to be effectively introduced into the IMS formeasurement. A unique, single pump low design is employed to both blowair through the nozzle ports, inhale the sampled gas into the IMS inlet,and to provide drift gas flow for the IMS. Typical flow rates are about800 to 1200 cc/min exhalation low for the nozzle, 150 cc/min inhalationlow through the inlet piece, and about 50 to 70 cc/min drift low. Acalibrated vent (not shown) is used to make up the difference in flowsand allow the pump to work without appreciable back pressure. For thelowest noise contribution, the pump is also controlled using a PWMcircuit.

ILLUSTRATIVE EXAMPLE Gridless IMS Design

Since the inner diameter of the guard rings is only 0.217 inches in theminiaturized design, grid screen structures are not necessary toestablish the field uniformly across the area of the ring normal to theion low. The ion reservoir is established in the region of the guardring above the control ring where the voltage potential is the lowestbetween control pulses.

A prototype of the instant IMS design was operated electrically suchthat the control function was operated only three guard rings down fromthe source. This had the effect of increasing the drift length byanother three guard rings and reducing the reaction region by the samelength. This lengthening of the drift region by approximately 35% shouldtheoretically cause no loss in ion current, since the ions are ravelingthe same overall distance, but will improve peak resolution. Since theMini-Cell concept employs the ion reservoir, there is not very muchgained by having a very long reaction region. The required reactantion/sample chemical reactions will occur for the most part in the ionreservoir region.

As can be seen from FIG. 15, a strong well-shaped reactant ion is theonly peak evident in the spectrum.

Also, eliminating the screen grid did not negatively affect theperformance of the IMS. Actually, since the optical transmission of agrid was only 61%, the actual performance was better, because more ionsreached the collector resulting in greater peak amplitude, thusimproving signal to noise. Because the ion reservoir technique enabledthe IMS to be efficiently miniaturized, the internal diameter becamesuch that the grids were not necessary to establish a uniform field onthe radius of the cell. Not using a complicated grid design greatlysimplifies the construction of the IMS and also virtually eliminatesmicrophonic noise pickup. The ion injection circuit can be thought of asusing a “virtual” grid to control the ion movement.

In accordance with FIG. 16, a handheld ion mobility spectrometer 160(IMS) for sampling a gaseous stream to detect trace chemicals is shown.The IMS includes a housing 161; a power connector 163A in communicationwith an on/off switch 163B which may be used to power the unit oralternatively to provide for charging of the on-board battery pack 164;an actuator trigger 165 for sampling initiation; an electrometer 166 forcontrolling the various processes necessary for operation of the IMS andfor calculating results to be forwarded to the LCD viewing screen 167;an ion detection circuit 168; an ion detection assembly detector housing162, including a sampling nozzle 169 in fluid communication with a gasflow pump, said sampling nozzle 169 including an inhalation inlet and atleast one exhaust nozzle port constructed and arranged to facilitateperturbation of a target surface; said gas flow sample pump 170 in fluidcommunication with said sampling nozzle 169, said gas flow sample pump170 constructed and arranged to provide exhaust air for low through saidat least one exhaust nozzle port, inhalation flow through saidinhalation inlet, and drift gas flow through said IMS; a drift cellconstruction (within the detector housing 162) in fluid communicationwith said sampling nozzle and including therein an ion reservoir inelectrical communication with an electronic ion injection controlcircuit; and an LCD viewing screen 167, or the like, for text display ofthe output results of the on-board processor; whereby sampling of agaseous stream is performed and any contaminants contained therein aredetermined and reported, e.g. via a USB port 171 to a remote PC.

All patents and publications mentioned in this specification areindicative of the levels of those skilled in the art to which theinvention pertains. All patents and publications are herein incorporatedby reference to the same extent as if each individual' publication wasspecifically and individually indicated to be incorporated by reference.

It is to be understood that while a certain form of the invention isillustrated, it is not to be limited to the specific form or arrangementherein described and shown. It will be apparent to those skilled in theart that various changes may be made without departing from the scope ofthe invention and the invention is not to be considered limited to whatis shown and described in the specification and any drawings/figuresincluded herein.

One skilled in the art will readily appreciate that the presentinvention is well adapted to carry out the objectives and obtain theends and advantages mentioned, as well as those inherent therein. Theembodiments, methods, procedures and techniques described herein arepresently representative of the preferred embodiments, are intended tobe exemplary and are not intended as limitations on the scope. Changestherein and other uses will occur to those skilled in the art which areencompassed within the spirit of the invention and are defined by thescope of the appended claims. Although the invention has been describedin connection with specific preferred embodiments, it should beunderstood that the invention as claimed should not be unduly limited tosuch specific embodiments. Indeed, various modifications of thedescribed modes for carrying out the invention which are obvious tothose skilled in the art are intended to be within the scope of theinvention.

The above examples are merely illustrative of the many applications ofthe system of present invention. Although only a few embodiments of thepresent invention have been described herein, it should be understoodthat the present invention might be embodied in many other specificforms without departing from the spirit or scope of the invention.Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention may be modifiedwithin the scope of the appended claims.

We claim:
 1. An ion mobility spectrometer (IMS) which incorporates anion reservoir for providing enhanced sensitivity, with an improvementcomprising; an electronic ion injection control circuit which enableshigh voltage switching in a manner effective to establish an ionreservoir, wherein said electronic ion injection control includes aresistive bridge circuit electrically coupled to a high voltagetransistor; and means for providing a low voltage trigger timing pulseeffective for tripping an opto-isolator, wherein said trigger timingpulse to said opto-isolator causes voltage to a base of the high voltagetransistor to vary with said trigger timing pulse and wherein said highvoltage transistor provides a sharp square wave voltage pulse to an ioncontrol ring effective to produce a resultant drop in voltage whichcauses ions in the ion reservoir to be injected into a drift region ofthe IMS along an ion drift tube.
 2. The ion mobility spectrometer ofclaim 1 wherein said electronic ion injection control circuit provides asufficiently high voltage to achieve a uniform control voltage radiallyacross a diameter of the ion drift tube thereby enabling elimination ofan ion control grid.
 3. The ion mobility spectrometer of claim 1 furthercomprising: a drift cell employing a hermetically sealed drift tubeconstruction using ceramic insulating rings joined to metal rings formedfrom a nickel-cobalt ferrous alloy by an active metal joining process,wherein the drift cell provides a self-enclosed cell construction,wherein said metal rings are constructed and arranged to be electricallycoupled to a high voltage control and electrometer board, and whereinsaid hermetically sealed drift tube prevents outside contaminants frombeing introduced therein.
 4. The ion mobility spectrometer of claim 3wherein said drift tube enables elimination of any screen grids.
 5. Theion mobility spectrometer of claim 3 further comprising a thin foilheater constructed and arranged to wrap around and heat the drift cell.6. The ion mobility spectrometer of claim 5, further comprising aninsulating member constructed and arranged to encase the drift cell andfoil heater.
 7. The ion mobility spectrometer of claim 6 furthercomprising a protective housing constructed and arranged to enclose theinsulating member encased drift cell and foil heater and to provideelectro-magnetic-force (EMF) shielding.
 8. The ion mobility spectrometerof claim 5, wherein said thin foil heater has two separate heater zoneswhich allow a body of the drift cell and gas inlet to be independentlyheated.
 9. The ion mobility spectrometer of claim 5, wherein said foilheater is controlled using a pulse-width-modulated voltage supply. 10.The ion mobility spectrometer of claim 3 wherein a radioactive sourceholder is provided for threadable engagement with a source fixtureassembly, thereby enabling manufacture of the drift cell withoutradioactive material therein.